A structure combined by sheet metal processing and welding of steel is common as a vehicle floor structure. The floor structure has a greatly complicated shape in order to install a reinforcing structure for increasing rigidity of a vehicle body and a space for housing a fuel pipe and an exhaust pipe at the bottom of the vehicle.
On the other hand, an approach in which a carbon-fiber-reinforced composite material is applied to the vehicle floor structure has been made, since a reduction in vehicle weight is strongly desired in order to reduce an environmental burden of a vehicle in recent years. The floor structure of the carbon-fiber-reinforced composite material described above is significantly lighter than an existing vehicle floor structure of steel, but the floor structure of the carbon-fiber-reinforced composite material is formed using expensive carbon fiber fabric and using a hand lay-up or autoclave method, which are methods with low productivity, to follow the complicated shape of the existing steel floor structure. Accordingly, the spread thereof is extremely limited from the viewpoint of economic efficiency. Recently, an approach to improve productivity has been made using a resin transfer molding method (RTM method) or the like (refer to Patent Document 1), but it is hard to say that the technology can be applied to general vehicles, since a curing reaction time of a thermosetting resin used as a matrix is rate-limiting.
A fiber-reinforced thermoplastic composite material in which a thermoplastic resin is used as a matrix have been developed as means for improving productivity of the composite materials. It is possible to give a fiber-reinforced thermoplastic composite material a shape in a short tact time by stamping molding after the fiber-reinforced thermoplastic composite is plasticized by heating. The necessary pressure for the stamping is lower than stamping molding of steel, and thus integral molding is possible if the size is one such as a vehicle floor structure. It is also possible to give the fiber-reinforced thermoplastic composite materials a shape of a hollow structure or an overhang, but for this, a mold having a complicated mechanism is necessary. Therefore, a simple shape which can be molded in a normal male-female shape is preferable in order to increase productivity.
Moreover, as another approach for reducing the environmental burden of a vehicle, the current to switch from an internal combustion engine to an electric engine as a power source is accelerated. At this time, it is necessary to mount a battery as an energy source. Since energy density of the battery is low in comparison with gasoline or diesel, there is a disadvantage of increasing size and weight of the battery. On the other hand, a fuel pipe or an exhaust pipe specific to the internal combustion engine is unnecessary, and thus the shape of the vehicle floor structure is greatly simplified.